1. Technical Field
The present invention relates to an image forming apparatus such as copiers, facsimiles and scanners.
2. Description of the Related Art
In electrophotographic image forming apparatuses, an image control method detecting toner adherence quantity with an optical sensor is widely used as an image density guarantee of produced images. This method includes changing image forming conditions (development potential or LD writing density) to form plural reference patches on a photoreceptor or an intermediate transfer belt, and irradiating infrared LED light to the reference patches. The method further includes detecting reflection light (regular or irregular reflection light) with an optical sensor such as photodiodes and phototransistors, and converting the detection results to toner adherence quantities of the individual tone patches. Then, the toner adherence quantity relative to the potential is plotted to do linear approximation. This slope of the line is development γ. An image density control controlling image forming conditions (development potential) to obtain a targeted adherence quantity using the approximation straight line is known.
Japanese published unexamined application No. JP-2005-308833-A discloses a method of detecting a halftone patch with an optical toner concentration sensor and a patch image area with an imaging apparatus to form an image without background fouling and blurred edge. Japanese published unexamined application No. JP-2000-147848-A discloses a method of correcting a charge bias based on information detected by a temperature and humidity sensor to optimally keep a background potential.
However, when a charging bias is determined so as to have a predetermined background potential after developing bias is determined so as to have a potential calculated as above, a correlation between the charging bias applied at a charging position and a charging potential remaining at a developing position changes when a surface potential of a photoreceptor varies due to time or an influence of environment. Therefore, when the charging bias is not properly set, the narrow background potential causes background fouling, and the wider background potential causes a blurred edge. A method of lowering the charging bias to the developing bias to form and detect a background fouling pattern and forecasting the background potential from the background fouling quantity is disclosed. However, the method forms background fouling on the whole surface of a photoreceptor in the main scanning direction, resulting in increase of toner consumption.
The method disclosed in Japanese published unexamined application No. JP-2005-308833-A forms plural patch patterns, and therefore insufficiently controls a toner field, does not disclose a method of precisely controlling the background potential with only a P sensor, and the imaging apparatus is expensive. The method disclosed in Japanese published unexamined application No. JP-2000-147848-A is difficult to grasp the surface potential properties of the photoreceptor with only an environment sensor, and the background potential is not properly maintained depending on the usage environment.